JP2672108B2 - Concentrated evaporator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a concentrating evaporator, for example, a concentrating evaporator suitable for concentrating evaporation of a nitric acid waste liquid containing a large amount of radioactive substances generated in a nuclear fuel reprocessing plant. .

[Prior Art] A nitric acid-concentrated evaporator for high-activity nitric acid waste liquid generated in a conventional nuclear fuel reprocessing plant is a BNFL version “Highly active liquid waste, evaporation and storich” (Highly active ligu).
id waste, Evavoration and storege), which is as follows.

The purpose of the nitric acid concentrating evaporator is to concentrate nitric acid waste liquid and recover nitric acid.To achieve this purpose, high-temperature steam is passed through the heating jacket around the tank and the heating coil in the tank to heat them. By doing so, the nitric acid solution in the tank is evaporated and concentrated. Since it handles high-concentration nitric acid, the pressure in the tank is set to about 50 mmHg to 70 mmHg to reduce the temperature in order to reduce the corrosion conditions.

Now consider the heating coil in the tank.
This coil has a portion effective for evaporating the liquid by being directly immersed in the liquid in the tank and a portion exposed to the gas phase region and not involved in heating the liquid. The temperature of the surface of the coil exposed in this vapor phase region rises to around 130 ° C, which is close to the temperature of the water vapor flowing in the coil, and the temperature in the liquid phase region is about 60 ° C at the most, which corresponds to the boiling point of nitric acid. Corrosion conditions are considerably more severe than those of the coil part, which is one of the important factors that determine the life of the evaporator. That is, since corrosion is also a type of chemical reaction, its rate increases with increasing temperature. In a nitric acid solution containing a large amount of fission products, the increase in corrosion rate with increasing temperature is as shown in Fig. 3. Particularly, because of this, the part of the heating coil exposed to the gas phase region and having a higher temperature than the coil part in the liquid phase region is particularly susceptible to corrosion.

In Japanese Unexamined Patent Publication No. 60-4898 "Single-body waste liquid concentrator",
After the concentrated solution is discharged, the built-in spray is used to wash away the corrosive components adhering to the inner surface of the evaporator, thereby preventing the corrosion of the members and prolonging their life.

However, this method has the drawbacks that it cannot be used during operation and that it does not prevent corrosion of the exposed portion of the heating coil due to high temperature.

[Problems to be Solved by the Invention] As described above, in the conventional nitric acid-concentrated evaporator,
The heating coil exposed to the gas phase region above the nitric acid has a problem that the corrosive environment is severe because its surface temperature becomes high, and in the cleaning method of the inner surface deposit described in JP-A-60-4898, the above heating There is a problem that the corrosion of the exposed portion of the coil cannot be reduced during operation.

The present invention aims at applying the above-described conventional concentrated evaporator to the present invention, and even when the concentrated evaporator is in operation, it exists in a gas phase region where the corrosion condition on the liquid surface in the concentrated evaporator is severe. The purpose is to significantly reduce the corrosion of the heating coil.

[Means for Solving the Problem] The above-mentioned object is to provide a tube-shaped heating coil in which steam for heating flows inside, over the upper and lower portions of the liquid surface of the process solution in the concentration evaporator. The heating coil located in the vapor phase region above the liquid surface of the process solution in the evaporator can be surrounded by a shield tube and the space between them is evacuated, or the vapor evaporated from the process solution in the can is condensed. This is accomplished by pouring the liquid onto the surface of the heating coil located in the gas phase region, or a combination thereof.

[Operation] According to the present invention, the temperature rise of the exposed surface of the heating coil in the vapor phase region where the corrosion conditions are severe in the concentrating evaporator can be reduced, and the corrosion of the heating coil can be significantly reduced. Further, the ceramic coating itself has a corrosion resistance effect.

[Example] When a nitric acid solution containing fission product ions, particularly ruthenium, cerium, and chromium is concentrated, the corrosive environment of the heating coil of the nitric acid-concentrated evaporator becomes extremely sensitive to temperature.
This is because ruthenium, cerium, and chromium are in a nitric acid solution, and more expensive ruthenium tetrachloride, tetravalent cerium, 6
It is susceptible to oxidation to valent chromium, which also corrodes the material. Fig. 4 shows the production rate of ruthenium tetroxide at 60 ℃.
However, it was found that when the temperature increased from 60 ℃ to 100 ℃, the production amount increased 200 times and the corrosion rate increased 10 times or more. 4
It was confirmed that the amounts of valent cerium and hexavalent chromium produced were several times higher. In the decompressed nitric acid concentrated evaporator, the temperature of the nitric acid solution is constant at 60 ° C, so there is no problem of corrosion of the heating coil part in the liquid, but the heating coil in the gas phase region above the liquid surface. The outer surface of the portion reaches a temperature of about 100 ° C., and the liquid in the can adheres to the outer surface as bumping or droplets, where the above-mentioned expensive ions are produced. Therefore, the corrosion rate of the heating coil portion in the gas phase region is several tens of times that of the heating coil portion in the liquid, and the damage due to the corrosion becomes a very serious problem.

As a drastic measure against this problem, it is optimal to have a structure with no heating coil in the gas phase region where the corrosion conditions are severe.The structure is such that the heating coil enters the liquid in the can directly from the side of the can. However, in order to obtain such a structure, a welded portion between the can and the heating coil is provided at a portion which is in direct contact with the high-level radioactive waste liquid containing a large amount of fission products. This is necessary and is extremely problematic in terms of device reliability.

Therefore, the present invention reduces the temperature of the heating coil surface existing in the gas phase region where the corrosion condition in the can is severe by a simple structure and a method which does not pose a serious risk to the apparatus, and the heating coil surface temperature is reduced. Is about the same as the temperature of the liquid in the can, and its corrosion is to be greatly reduced.

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1 and 2. As shown in FIG.
A tubular heating coil 1, a heating coil shield tube 2 in the gas phase region, an evaporator main body (tank) 3, a heating jacket 4 around it, a heating coil vapor inlet nozzle 5,
It is composed of a heating coil vapor outlet nozzle 6, a heating jacket vapor inlet nozzle 7, and a heating jacket vapor outlet nozzle 8, and a process solution (nitric acid waste liquid) 9 is supplied into the tank 3. One part of the heating coil 1 is immersed in the process solution 9 in the tank 3, and the other part is in the vapor phase region above the process solution 9.

The heating coil 1 and the heating coil shield tube 2 have the structure as shown in FIG. 2 in the above vapor phase region, that is, the heating coil shield tube 2 is covered on the outside of the tubular heating coil 1. There is a space 11 between them. Further, steam for heating flows through the inside 10 of the heating coil 1. The heating coil 1 and the heating coil shield tube 2 are sealed to each other at a location inside the process solution 9 and at a location outside the evaporator main body 3, whereby
The space 11 between them is sealed. The space 11 has a vacuum.

Next, the operation will be described. First, the process solution 9 (nitric acid solution in this embodiment) is supplied into the evaporator main body 3. After that, the inside of the evaporator main body 3 is operated under a reduced pressure of about 50 mmHg. The space 11 between the heating coil 1 and the heating coil shield tube 2 is in a vacuum. Here, heating steam of about 130 ° C. is introduced into the heating coil 1 and the heating jacket 4 from the heating coil steam inlet nozzle 5 and the heating jacket steam inlet nozzle 7. The water vapor heats the process solution 9 in the evaporator main body 3 to evaporate and concentrate it to a desired concentration. At the time of this evaporation / concentration, the portion of the heating coil 1 in the gas phase region is shielded from the corrosive atmosphere in which corrosive fluid vapor exists by the heating coil shield tube 2. The steam for heating passes through the heating coil 1 and the heating jacket 4, and then the heating coil steam outlet nozzle 6
And it goes out from the heating jacket steam outlet nozzle 8. The heating steam continues to flow in the heating coil 1 and the heating jacket 4 until the process is completed.

With the above configuration, the following effects can be obtained. First, as a first effect, the heating coil 1 in the vapor phase region, which has been conventionally exposed, is covered with the heating coil shield tube 2, so that the heating coil 1 can be shielded from the corrosive atmosphere. Secondly, by making the space between the heating coil and the heating coil shield tube 2 into a vacuum, the heat insulating effect between the heating coil 1 and the heating coil shield tube 2 becomes large. The surface temperature of the heating coil shield tube 2 is considerably lower than the temperature, and the corrosion condition of the surface of the heating coil shield tube 2 is considerably relaxed as compared with that of the conventional heating coil surface.

An example of surface temperature calculation is shown below. The conditions are (1) inside diameter of heating coil 1 19 mm, outside diameter 25 mm, without heating coil shield tube (conventional case), and (2) inside diameter of heating coil 1 19 mm, outside diameter 25 mm, heating coil shield tube 2 The case where the inner diameter is 44 mm and the outer diameter is 50 mm (in the case of the present embodiment). In each case, steam at 130 ° C. was flowing in the heating coil 1, and in the case of (2), a vacuum was set between the heating coil 1 and the heating coil shield tube 2. The material of the heating coil 1 and the heating coil shield tube 2 was SUS304. Steam heating tube 1
Heat transfer coefficient h _{1 to} the coil, heat transfer coefficient h ₂ from the coil to nitric acid vapor,
The thermal conductivity λ in the coil is as follows.

h ₁ = 1.63w / m ² · k h ₂ = 11.63w / m ² · k λ = 16.5w / m · k In addition, heat transfer in the vacuum section is limited to radiation.

 The calculation results are shown below.

In the case of (1): The surface temperature of the heating coil is 126.9 ° C. In the case of (2): The surface temperature of the heating coil shield tube is 60.4 ° C. According to Fig. 3, in the case of (2), compared to the case of (1). The average metal loss rate is about 1/100.

Due to the above two effects, one of the major rate-determining conditions for achieving a long life of the concentrated evaporator, the problem that the corrosion on the heating coil surface in the gas phase region is larger than that in the liquid phase region is
It is greatly alleviated, and the life of the concentrated evaporator can be extended.

It is more effective if a layer that reflects radiation is provided on the inner surface of the heating coil shield tube 2.

A second embodiment of the present invention will be described with reference to FIG. In the present embodiment, the exposed portion of the heating coil is cooled by intensively contacting the condensate of the vapor evaporated from the concentrating evaporator with the exposed portion of the heating coil. FIG. 5 shows a nitric acid concentrated evaporator. As for the constitution, by putting the nitric acid solution 9 in the evaporator 12 and heating and evaporating it by the heating jacket 4 and the heating coil 13 through which the steam for heating flows,
The nitric acid solution is concentrated by evaporation. Steam as a heating source (130
(° C.) enters the heating coil 13 through the heating steam inlet 15 and exits through the heating steam outlet 16. The nitric acid solution 9 is vaporized by heating, passes through the gas passage holes 17 in the condensate return plate 14, passes through the mist separator 18, and exits from the evaporative gas outlet 19 to proceed to the next process. The evaporative gas outlet 19 is also connected to a decompression device at the same time, and the pressure inside the evaporating can 12 is maintained at 50 to 200 torr. Part of the evaporative gas from the nitric acid solution is a mist separator
It is cooled in 18 and becomes a condensate and returns to the inside of the evaporation can 12. At this time, in the present embodiment, this condensate is once captured by the condensate return plate 14 and vaporized above the liquid level in the can 12. Falling contact is made on the heating coil 13 in the phase region. As a result, the portion of the heating coil 13 existing in the vapor phase region is constantly in contact with the condensate at around 60 ° C., and the surface temperature thereof is almost the same as that of the condensate, which significantly reduces the corrosive environment. It becomes possible.

Although two examples have been described above, these examples are not limited to the case where the concentrated and evaporated process solution is nitric acid, and can be applied to the case where hydrochloric acid, sulfuric acid, phosphoric acid, or another aqueous solution.

It should be noted that an embodiment combining the ideas of the above embodiments is also possible. For example, in FIG. 5, the heating coil 13 may be a heating coil with a shield tube as described in the first embodiment, and the condensate may be poured onto the surface of the shield tube.

[Effects of the Invention] According to the present invention, it is possible to significantly reduce the corrosion of the heating coil portion in the vapor phase region where the corrosion conditions in the waste liquid concentrating evaporator are severe, and to greatly improve the durability.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a first embodiment of a concentrating evaporator according to the present invention, and FIG. 2 is a perspective cross-sectional view showing a double pipe structure including a heating coil and a heating coil shield tube in FIG. Figure 3 shows SUS depending on the temperature in a nitric acid vapor atmosphere.
Fig. 4 is a graph of the average thickness reduction rate ratio of Fig. 4, Fig. 4 is a relationship diagram between the production ratio of ruthenium tetroxide and temperature, and Fig. 5 is a sectional view of a nitric acid concentrating evaporation pipe according to a second embodiment of the present invention. 1 ... Heating coil 2 ... Heating coil shield tube 3 ... Evaporator body 4 ... Heating jacket 5 ... Heating coil vapor inlet nozzle 6 ... Heating coil vapor outlet nozzle 7 ... Heating jacket vapor inlet nozzle 8 ... … Heating jacket Vapor outlet nozzle 9 …… Process solution, 10 …… Steam passage for heating 11 …… Vacuum space, 12 …… Main body of nitric acid evaporator 13 …… Heating coil, 14 …… Condensate return plate 15 …… Heating Steam inlet, 16 …… Heating steam outlet 17 …… Gas passage hole, 18 …… Mist separator 19 …… Evaporated gas outlet

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masato Miyazaki, 1-1 1-1 Saiwaicho, Hitachi, Ibaraki Prefecture Hitachi factory Hitachi Ltd. (72) Masayuki Saito 3-1-1, Saiwaicho, Hitachi, Ibaraki No. 1 inside Hitachi Works, Hitachi Ltd. (72) Inventor Shinichi Watanabe 3 1-1 No. 1 Sachimachi, Hitachi City, Ibaraki Inside Hitachi Works, Hitachi Ltd. (56) Reference JP-A 63-62590 (JP, A) ) JP-A-57-128897 (JP, A) JP-A-61-120999 (JP, A) JP-A-55-39229 (JP, A) JP-A-1-121663 (JP, A) JP-A-57- 105279 (JP, A)

Claims (3)

(57) [Claims] 1. A process in a concentrating evaporator is provided with a tubular heating coil in which steam for heating flows inside, over a region above and below a liquid surface of a process solution in the concentrating evaporator. A condensing evaporator, wherein the heating coil located in the gas phase region above the liquid surface of the solution is surrounded by a shield tube and the space between the two is evacuated. 2. A process in a concentrating evaporator is provided with a tubular heating coil in which steam for heating flows inside, over a region above and below a liquid surface of a process solution in the concentrating evaporator. Means for capturing the concentrated liquid of vapor evaporated from the solution, and means for pouring the concentrated liquid on the surface of the heating coil located in the gas phase region above the liquid surface of the process solution in the can Concentrated evaporation can, which is characterized. 3. The method according to claim 1, further comprising means for capturing a condensate of vapor evaporated from the process solution in the concentrating evaporator, and means for pouring the condensate on the surface of the shield tube. A concentrated evaporation can.